Particulates, particularly ultrafine particulates of titanium oxide have very wide application areas in the industrial field and their diversified uses include ultraviolet light-shielding materials, additives to silicone rubber, photocatalysts and the like. The titanium oxide is referred to as “titanium dioxide” in Japanese Industrial Standard (JIS) but the term “titanium oxide” is used as a common name. Accordingly, this simple term “titanium oxide” is hereinafter used in the present invention. The importance of titanium oxide is increasing in the use for shielding ultraviolet light, for example, in the field of cosmetics, clothing and the like. As a shielding material, ultrafine particulates of titanium oxide are being used in many cases because of its high safety. For the shielding, two functions of absorbing and scattering the ultraviolet rays are necessary. The ultrafine particulates of titanium oxide have both of these functions.
Titanium oxide has three crystal forms, i.e., brookite, anatase, and rutile, the latter two of which are very important for industry. Because the band gap (corresponding to excitation energy) of rutile is lower than that of anatase (i.e., the optical absorption wavelength range is on the longer wavelength side than anatase), rutile has been considered to be preferable for ultraviolet light-shielding use. However, in actual ultraviolet light-shielding uses, a scattering effect depending on particle diameter, as well as absorption, has to be dealt with.
Recently, it has been reported that titanium oxide has a property of absorbing ultraviolet light at a wavelength of about 400 nm or less to excite the electrons in the outermost shell, allowing the generated electrons and holes to reach the surface of particulates, where they combine with oxygen or water to generate various radical species, thereby decomposing organic materials that exist near the surface of the particle. Therefore, in the case of using titanium oxide in cosmetics and the like, generally it has been widely attempted to practice surface treatment on the surface of particulate, particularly ultrafine particulate titanium oxide.
The fine particulates of titanium oxide are also used for making use of the photocatalytic reaction resulting from photoexcitation of titanium oxide. Furthermore, where titanium oxide is used for scattering ultraviolet light, ultrafine particulates of titanium oxide having a primary particle size of about 80 nm are used. Generally, the primary particle diameter of ultrafine particulates has not been defined. However, usually, those fine particulates having about 0.1 μm or less are referred to as ultrafine particles.
The production processes for titanium oxide are roughly divided into liquid phase processes where titanium tetrachloride or titanyl sulfate is hydrolyzed in a hydrophilic solvent and vapor phase processes where a volatile material such as titanium tetrachloride is vaporized and then the resulting vapor is reacted with an oxidizing gas such as oxygen and steam. In the vapor phase process, ultrafine particulate titanium oxide is obtained. However, only titanium oxide composed of anatase, as a main crystal form, has been obtained. Therefore, conventionally, ultrafine particulate titanium oxide of a rutile structure has been obtained by a liquid phase process.
In general, the powder of titanium oxide produced by the liquid phase process disadvantageously undergoes heavy aggregation. For this reason, when titanium oxide is used in cosmetics and the like, the titanium oxide must be strongly cracked or pulverized, so that there arise problems such as mingling of abraded materials attributable to the pulverization treatment or the like, non-uniform distribution of the particle size, or a poor feeling.
Several production processes for titanium oxide having high rutile contents have heretofore been proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 3-252315 discloses a production process where the ratio of hydrogen in the mixed gas comprising oxygen and hydrogen in the vapor phase reaction is changed to adjust the ratio of the rutile content and a process for producing high purity titanium oxide having a rutile content of 99% or more by adjusting the concentration of hydrogen to from 15 to 17% by volume. Also, Japanese Unexamined Patent Publication (Kokai) No. 6-340423 discloses a production process for titanium oxide having high rutile content (the rutile content being from 85% by weight to 90% by weight) where the production is performed by setting the molar ratio of titanium tetrachloride, hydrogen and oxygen in the mixed gas to specified mixing ratios.
In the case of titanium oxide produced by the vapor phase process, the same problems as in the production by the liquid phase process will arise. That is, although particulates, particularly ultrafine particulates of titanium oxide may be obtained by the conventional vapor phase process, only particulates of titanium oxide which have undergone grain growth can be obtained. Thus, for obtaining ultrafine particulates of titanium oxide, the titanium oxide must be strongly cracked or pulverized. Moreover, in titanium oxide having high rutile content, the ultrafine particulates, although called ultrafine particulates, do not have sufficient specific surface area and are insufficient in dispersibility, a quality which is desired in various uses such as cosmetics.